For many years, the oil companies have equipped the drill strings with various bottom drill bits which supply, in the course of drilling and without interrupting the latter, various measurements suitable for carrying out a deviation or for evaluation of the layers traversed and, more recently, data such as the torque and the weight transmitted to the drill bit as well as an evaluation of the vibratory behaviour of the string and of the drill bit sensed at the bottom.
However, the technological range of some of these drill bits remains limited; in effect, the current techniques for information transmission, in the course of drilling, by sending pulses into the mud or by sending electromagnetic waves allow only a low data throughput from the bottom to the surface, which considerably reduces the development of more sophisticated drill bits.
However, for several years, the oil industry has started to use electrical links arranged from the surface to the bottom of the well, along the drill stem, and intended, among other things, to provide the power supply to a motor situated at the bottom. Such links are described in the following documents: U.S. Pat. Nos. 3,378,811, 3,518,608, 3,696,332, 3,807,502, 3,825,078.